1. Field of the Invention
The present invention relates to an optical path adjusting device for shifting an optical path of a light beam coming out of the optical path adjusting device in parallel with an original optical path of the light beam, and more particularly, to an optical path adjusting device equipped with an optical parallel operative to cause a displacement of an optical path of a light beam coming out of the optical path adjusting device from an original optical path and vary a vertical position of the light beam with respect to the original path.
2. Description of the Related Art
In order for an optical instrument to direct a light beam from a light source to a specific optical element, it is necessary to adjust an optical path of the light beam so that the light beam impinges on the specific optical element. For example, there have been well known a variety of image forming machines, such as copy machines, printers and facsimile machines, for forming a subject image on a recording paper. Such an image forming machine is equipped with an optical scanning device operative to scan an image carrier such as a photosensitive drum with an image information bearing scanning beam such as a laser beam emanating from a semi-conductor laser source. The optical scanning device has a scanning optical system comprising a rotary polygon mirror operative to reflect and deflect an image information bearing scanning beam and an fθ lens system which performs regulation of the scanning beam after deflection necessary to form an image without producing distortion, and reflection optical means for directing the scanning beam to an image carrier rotating in a specified direction such as a rotary photosensitive drum so as to scan the photosensitive drum with the scanning beam, thereby forming an electrostatic latent image on the photosensitive drum. The electrostatic latent image on the photosensitive drum is developed as a toner image and then transferred to a printing medium such as a printing paper.
In order for the optical scanning device to scan the photosensitive drum precisely, it is essential to adjust an optical path of scanning beam so that the scanning beam impinges on the polygon mirror in a specified position. For the adjustment of optical path of scanning beam, it has been known to use an optical parallel such as disclosed in, for example, Japanese Unexamined Patent Publication No. 63-101807, entitled “an optical path adjusting device”. One of the prior art optical parallel adjusting devices is shown in FIG. 8.
Referring to FIG. 8, the optical parallel 81 made of a transparent glass having an appropriate refractive index is mounted on a support frame 82. The support frame 82 is pivoted by a pivot shaft 82a so as to swing about a pivotal axis Y intersecting an optical path of scanning beam S passing through the optical parallel 81. When swinging the support frame 82 through a certain angle, an angle of the scanning beam incident upon the optical parallel 81 changes according to the angle of swing, so that the scanning beam after passing the optical parallel 81 travels in an optical path shifted in parallel with an optical path in which the scanning beam before passing the optical parallel 81 travels.
One type of image forming machine equipped with an optical scanning device employs a plurality of scanning beams and reflection and deflection means common to the scanning beams. Another type of image forming machine equipped with an optical scanning device, which is called a tandem type image forming machine, employs a plurality of scanning beams and a plurality of reflection and deflection means one for each scanning beam. The former type is advantageous to compactness and simple controllability of the image forming machines. For example, such a color image forming machine for recording a color image is typically equipped with an optical scanning device that uses four scanning beams, namely a yellow (Y) image information bearing scanning beam, a magenta (M) image information bearing scanning beam, a cyan (C) image information bearing scanning beam and a black (BL) image information bearing scanning beam with which a photosensitive image carrier is scanned to form an electrostatic latent image thereon. Another image forming machine is provided with an optical scanning device which uses two scanning beams with an intention to form an image quickly. The two scanning beams scan a photosensitive image carrier simultaneously but at different positions, respectively, so as to enable quick image forming.
In the image forming machine which employs a plurality of scanning beams, an electrostatic latent image formed on the image carrier is made blurred and distorted if the scanning beams are out of specified relative scanning position on the image carrier. Therefore, it is essential for the image forming machine which employs a plurality of scanning beams to adjust optical paths of scanning beams precisely. However, it is troublesome to adjust an optical path of scanning beam by the prior art optical parallel adjusting device. As shown in FIG. 8, because the pivotal axis Y of the pivot shaft 82a intersects, directly or spatially, the optical path of scanning beam S, an incident angle of scanning beam upon the optical parallel 81 changes greatly even for a small angle of swing of the optical parallel 81. In addition, because a change in the incident angle of scanning beam upon the optical parallel 81 depends only upon an angle of swing of the pivot shaft 82a, it is difficult to achieve precise adjustment of an incident angle of scanning beam upon the optical parallel 81 by the prior art optical parallel adjusting device.
Although the pivot shaft 82a is secured to a housing frame of the optical scanning device by an adhesive after the adjustment of incident angle of scanning beam upon the optical parallel 81 in order to fix the incident angle of scanning beam, the adhesive possibly comes off from the pivot shaft 82a due to vibrations while the image forming machine operates after a lapse of long time. In the event of exfoliation of the adhesive, the optical parallel 81 easily and accidentally swings, so that the incident angle of scanning beam upon the optical parallel 81 changes. The accidental swing of the optical parallel 81 results in producing of a blurred image or an unclear image.